A conventional example of a stage mechanism, which has a plate-like member as movable portion to align the movable portion at a predetermined position in an in-place direction, is disclosed in a patent publication (Japanese Patent Laid-Open No. 8-006642). In the disclosed arrangement, only a plate-like member which holds a workpiece is two-dimensionally moved using a plane pulse motor as a driving source. This method moves only the plate-like member as the movable portion and, thus, is suitable for reducing the size and weight of a stage movable portion.
FIG. 24 is a view showing the arrangement of a movable element in a conventional stage mechanism. An air supply hole 60 and units 52, 54, 56 and 58 each having coil-wound inductor toothed cores are formed in a box-like member 22 to constitute a movable element.
FIG. 25 is a view showing the arrangement of part of a stator in the conventional stage mechanism. The stator is formed as a plate-like member 24, and magnetic projections 72 are two-dimensionally formed on a magnetic base 70. A space between the projections 72 is filled with a resin 74.
FIG. 26 is a sectional view of the stator and movable element in the conventional example. The movable element further comprises permanent magnets 80 and 82, each of which serves as the thrust source of the movable element and at the same time generates a suction force between the movable element and the stator. Air supplied from the air supply hole 60 of the movable element generates an air pressure between the movable element and the stator. The air pressure and suction force become well balanced, thereby allowing the movable element to levitate above apart from the stator.
The units 54 and 56 each having coil-wound inductor toothed cores generate a thrust in the first direction while the units 52 and 58 generate a thrust in the second direction perpendicular to the first direction due to Sawyer's principle. This enables the movable element to freely generate a thrust in the first or the second direction and move two-dimensionally.
However, in the conventional arrangement, the movable element levitates by the air pressure and cannot be used in a vacuum. Also, the stage mechanism is driven by a pulse motor scheme based on Sawyer's principle, and vibrations are likely to be transmitted from the stator both in magnetic gap and traveling directions. This makes it impossible to perform high-accuracy position control.
The movable element includes coils and trailing power cables for supplying power. For this reason, the movable element is susceptible to disturbance by the cables. This also makes it impossible to perform high-accuracy position control.
In addition, the arrangements shown in FIGS. 24 to 26 can generate a thrust only in the first and second directions perpendicular to each other of the in-place direction and cannot perform tilt correction and position control in a rotational direction to control the posture of the stage mechanism. For this reason, they cannot be used in an apparatus which requires a precise posture and high alignment accuracy, such as a semiconductor exposure apparatus. If they are used, a fine adjustment stage needs to be separately provided to control the posture in the rotational direction. This increases the complexity of the stage mechanism.